CRUSHING AND GRINDING SPEED OF MILL FOR BEST THEORETICAL EFFICIENCY*
Internal diameter Proportion of mill volume occupied by charge at r
feet 0.1 0.2 0.3 0.4 0.5 0.6
1 59.15 60.45 61.82 63.36 65.10 67.01
2 41.88 42.75 43.72 44.81 46.04 47.40
3 34.19 34.90 35.69 36.58 37.58 38.69
4 29.61 30.22 30.91 31.67 32.55 33.51
5 26.48 27.04 27.64 28.34 29.11 29.96
6 24.18 24.69 25.24 25.88 26.58 27.36
7 22.38 22.85 23.36 23.94 24.60 25.32
8 20.94 21.38 21.86 22.40 23.02 23.70
9 19.74 20.15 20.60 21.13 21.70 22.34
10 18.72 19.12 19.56 20.04 20.59 21.20
* Davis, Trans. A. I. M. M. E.
Probably the most important difference of opinion in ball- and tube-mill operation is in the proper size of feed to be supplied. Extremes are represented by_the Inspiration plant, Arizona, on one hand, where 3-in. material is reduced at one operation to —48 mesh in a large diameter mill, and the Canada Copper Corporation, on the other hand, where crushers and rolls are used to bring the ore down to 10 mesh, after which two-stage ball-mill reduction carries it to —100 mesh. The tendency of modern operations is to save power by crushing fine before introducing the material into the ball mills.
An important modification of the tube-mill is the Hardinge mill, in which the discharge end is cone-shaped. The Hardinge modification is based on the theory that this construction leads to a classification of balls in the mill, the small balls or pebbles taking a position near the discharge where the material being treated is also finest. Theoretically, then, the material being pulverized and the pulverizing medium are proportioned in size in this mill with a resulting operating economy.
Primary Breaking.—Primary breaking, in the usual acceptance of the expression, means the first stage of rock reduction, in which the rock from its original source is given its first operation of size reduction. Primary breaking usually refers to the first operation at the particular installation where the work is being done, consequently it may vary in its requirements from the ability to receive pieces 6 or 7 ft. in diameter to machines required to accept only 8- to 12-in. pieces. In general, however, it is most convenient to consider primary or coarse breaking as the field in which the rock or ore directly coming from quarry or mine is fed into the machinery providing the first breaking state.
In large quarrying operations, the rock as it is blasted from the solid, ordinarily contains pieces too large to enter any breaker yet built. These pieces have to be drilled and blasted so as to secure sections that will go into the machines available. Jaw breakers have been built capable of receiving 7-ft. pieces of rock, and gyratory breakers capable of receiving 5-ft. pieces. There is no reason why larger machines should not be constructed, except that they are very expensive and their tonnage